The history of the Jews, who went down into Egypt and came out from Egypt, seems to accord with this. The years they spent there were 430, but not all of them lived a hundred years and more, as did Moses and Joshua, so that the time of the fourth generation would be appropriate in this context. So let us search rather for a mystical sense. In fact, the number four adapts well to all numbers, and it is in a certain sense the root and base of the decimal. It also represents the midpoint of the number seven. In fact, the ninety-third psalm is entitled “fourth day of the week” because this number is the intermediary between the first three and those that follow. In fact, three days precede it: the first, the second and the third; and three follow: the fifth, the sixth, the seventh. One who sings this psalm is proceeding through the life of this world, so to speak, in accordance with aptly placed numbers, like a quadrangle stable and perfect. In four books the Gospel is complete and perfect. There are four mystical animals;14 and there are also four parts of the world, from which the assembled children of the church have propagated the most holy kingdom of Christ, coming from east and west and north and south. The holy church, therefore, has arisen with four sides. The decade too derives from this number. For if you total up the numbers from one to four you will have the number ten. Count one, add two to this: this makes three. Add three to three, this makes six. And to six add four, and this makes ten. Four then generates the decade, and the decade includes all numbers. Four is also the number of ages of a man: childhood, adolescence, virility and maturity. He rises gradually, and his wisdom is consolidated. Thus the fullness of wisdom comes, considering the ages, in fourth place. For this reason even if one has formerly been subjected to the king of Egypt, nevertheless with the age of maturity he is freed from his power and acknowledges his duty to follow the law. Then the sea of this life opens up to him.
After having said this of Abraham himself, God speaks of the children who will come from him: “In the fourth generation they shall come back here,” meaning the generation that would return to the land of inheritance. This is why he says that the return would take place after four hundred years, “because the iniquity of the Amorites is not yet complete”—iniquity for which they will suffer ruin, so that their condemnation will allow the descendants of Abraham to occupy their land. For God inflicts even chastisements with measure and in time, using patience until the time of retribution has arrived. There is a similar and edifying saying in the Gospel: “Then Jesus began to upbraid the cities where most of his mighty works had been done, because they did not repent: Woe to you, Chorazin! Woe to you, Bethsaida! For if the mighty works done in you had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes.” To which one might object: Why then were the miracles not done in Tyre and Sidon, because they would have repented, but were performed instead in places where the people did not repent? We would respond that the Son of God who acted in this way is Wisdom. As he knew the hidden things, he knew that these people would not have been authentically repentant, even while doing penance, and this is why the miracles did not take place among them. And one could appropriately say about these people: It was better for them not to have known the truth than, having once known it, to return to their former errors. Thus he did not do works in Tyre and Sidon, because their repentance would be fragile.… However, one might also ask whether this was not said by the Savior in a hyperbolic manner, simply to make those people reflect who had seen his miracles and had not repented, for hyperbole is a common teaching device.The patience and goodness of the judge are shown, then, in the fact that he waits until the sins of the Amorites have reached their full measure. It is only after reproaches, exhortations and everything that can provoke repentance that God inflicts chastisements. The same was true in the case of Pharaoh: often reprimanded and having obtained many reprieves, through his hardness of heart he brought upon himself the final judgment as well.
(Verse 16) But in the fourth generation they will return here. There is no doubt that these will be the ones who are descended from Abraham. It is asked how it is written in Exodus: In the fifth generation the children of Israel went forth from the land of Egypt (Exodus 15:18). On this chapter, we have published a small volume.
But in the fourth generation, they shall return here. Kohath entered Egypt with his father Levi, whose son was Amram, whose son was Aaron, whose son was Eleazar, who was the fourth from Kohath. And he went out from Egypt with his father Aaron, and when he died in the wilderness, he himself entered the Promised Land.
[AD 397] Ambrose of Milan on Genesis 15:16